The present invention relates to systems for controlling emission of fuel vapors from a motor vehicle fuel tank. Present regulations require that fuel vapor vented from the tank be contained and stored during periods of engine inoperation. Typically, the vapor is stored in a reservoir or canister filled with adsorbents such as granulated carbon or charcoal. Upon engine startup, an electronically controlled solenoid valve is operated to control flow of vapor from the storage canister to the engine air inlet. Atmospheric air is admitted to purge the canister through an electronic solenoid valve.
Heretofore, problems have been encountered in purging the canister during engine operation and drawing vacuum in the canister and the fuel tank in the event of failure of the solenoid valve in open position. During periods of engine inoperation ambient cooling of the fuel tank may also create vapor condensation and a vacuum in the tank, which can collapse the tank. Thus, it has long been desired to provide a way or means of controlling vapor flow in the system and preventing a vacuum in the fuel tank also to provide for proper vapor venting to the storage canister, particularly where vapor pressure exceeds a predetermined limit value in the tank during periods of vehicle operation in elevated ambient temperature conditions.
Furthermore, where a mechanism or valve is to be employed for controlling the flow of vapor to the canister and preventing such a vacuum, the mechanism must of necessity be of very low permeability so as to prevent migration or escape of fuel vapor over time to the atmosphere when the mechanism is permitting flow of vapor from the tank to the canister during periods of engine inoperation.